(1) Field of the Invention
This invention relates to a process for the preparation of selective light absorptive methacrylic resins. More particularly, it relates to a process for the preparation of methacrylic resins capable of selectively absorbing light rays having a wavelength of about 580 nm.
(2) Description of the Prior Art
Since methacrylic resins have an excellent transparency and a high strength, they are widely used in various fields, for example, as illuminators, signboards, glazings, filters and the like.
A demand for comfortable illumination has been increased with elevation of the living level and therefore, it is desired to improve the rendition in relation to color in a high-brightness lamp and prevent the glare in the field of illumination.
Furthermore, with recent development of office computers and word processors comprising a cathode ray tube (hereinafter referred to as "CRT" for brevity), the problem of the fatigue of operator's eyes by high-brightness letters or figures on the CRT in case of the long-time use of these instruments becomes serious.
It is known that the foregoing problems can be solved if a light source is viewed through a cover or filter capable of selectively absorbing light rays having wavelengths of about 580 nm which exhibit a highest visibility to men. As a material to be used for attaining this purpose, a glare-preventive glass comprising an inorganic glass coated with neodymium oxide having a high absorption to rays having wavelengths of about 580 nm is known (see Japanese Unexamined patent Publication No. 51-99544). Since this glare-preventive glass is formed by coating neodymium oxide on the surface of an inorganic glass, it is difficult to manufacture a glare-preventive glass article of an optional desired shape. Furthermore, the glare-preventive glass is heavy and easily borken while it is used. Glare-preventive filters comprising plastics instead of inorganic glass have also been investigated and proposed. For example, Japanese Examined patent Publication No. 40-3150 proposes a process in which a methacrylic resin is dipped in an aqueous potassium hydroxide solution in methanol and the resin is then dipped in an aqueous neodymium salt solution in methanol to form a coating on the surface of the resin. In this process, however, the coating effect is insufficient and the neodymium salt is readily separated. Japanese Examined patent Publication No. 42-3949 proposes a process in which a partially polymerized liquid of a starting methacrylic monomer material is cast in a mold the molding surface of which has been coated with an organic film of a water-soluble polyether polymer containing a neodymium salt, and the polymerization is effected in the casting mold. This process, however, has a problem such that incorporation of the neodymium salt into the resin is difficult and the preparation steps are complicated and costly. Moreover, Japanese Examined patent Publication No. 44-5091 proposes a process in which methyl methacrylate is incorporated with a neodymium solution in anhydrous stannic chloride solvent and the resulting solution is subjected to the cast polymerization. However, since anhydrous stannic chloride fumes in air to form hydrogen chloride gas, handling is difficult and a methacrylic resin sheet obtained by the polymerization tends to be readily yellowed. Still further, Japanese Examined patent Publication No. 57-59263 proposes a plastic composition comprising a plastic material and a neodymium compound incorporated therein. According to this proposal, the neodymium compound is blended with the plastic material obtained by the polymerization, or the plastic material obtained by the polymerization is dissolved in a solvent and the solution is mixed with the neodymium compound. However, the plastic material in the composition must have a relatively low degree of polymerization so that it can be subjected to molding or extrusion. Therefore, the shaped articles made therefrom have poor mechanical strength and solvent resistance.